docs(site): explain the hybrid algorithm and credit Matthew Heberger

[CooperBigFoot]

How it works + full Matthew Heberger credit (m5)

Fills the two m3 placeholder pages with real content: a plain-language explanation of
shed's hybrid delineation algorithm for hydrologists, and full credit to Matthew
Heberger, who originated the method.

What's here

• docs/how-it-works.md — no-CS-jargon walkthrough of the hybrid vector+raster
  method. Defines every domain term at first use (outlet/pour point, drainage
  basin/watershed, unit catchment, flow direction/D8, flow accumulation, snapping),
  contrasts the pure-raster (accurate but slow) and pure-vector (fast but coarse at
  the outlet) approaches, explains the hybrid idea (assemble upstream unit catchments
  by walking the river network; use the D8 grid to trim only the terminal home
  catchment; dissolve into one polygon), and maps it onto shed's named stages
  (resolve_outlet → traverse → pre_merge_units → refine → dissolve → compose_result).
• docs/credits.md — credits the algorithm to Matthew Heberger
  (ORCID 0000-0001-9122-0030): the 2025 paper (cited as a submitted manuscript /
  preprint with its stable SSRN DOI 10.2139/ssrn.5939056; the journal
  version-of-record is unpublished, so no volume/journal DOI is invented), the
  verbatim @software{delineator} BibTeX, the
  Global Watersheds web app, and the Djokic & Ye (1999) lineage note. Frames shed as
  an independent Rust reimplementation of his MIT-licensed method, generalized to any
  HFX-compliant hydrofabric (his delineator is MERIT-specific).

Verification

• uv run --project crates/python --only-group docs mkdocs build --strict exits 0
  with no cargo/maturin/GDAL activity (no target/), zero warnings, and both
  pages rendered.
• Only docs/how-it-works.md and docs/credits.md are changed.

Scope

• Touches only the two content pages. mkdocs.yml and all other files are untouched
  (m3 owns the nav; m4 owns the other content pages).